This invention relates to displays such as of planar form.
Plasma display panels can give a high resolution, color display and be relatively compact. Present plasma display panels, however, are relatively inefficient, with luminous efficiencies being below 1 lm/W, which is considerably less than that of a CRT of about 4 lm/W. Also, plasma displays need high striking voltages, which can only be produced by expensive driver electronics.
Existing plasma displays operate in the following way. The high voltage between the cathode and anode produces a cathode fall region in from of the cathode through which plasma ions are accelerated towards the cathode. The ions impact the surface of the cathode and their energy is dissipated into heat and into the production of secondary electrons, the yield of which is proportional to the work function of the cathode metal. The secondary electrons drift through the gas plasma making ionizing collisions with the gas atoms and thereby sustaining the gas plasma. The secondary electrons also excite neutral atoms to resonance states, the gas mixture being chosen to contain gas species with resonant levels in the violet to ultra-violet (VUV) range of the spectrum so that, as the atoms fall back to their neutral state, they give up their energy as radiation in the VUV range. Phosphors in the display convert the VUV to visible light through the mechanism of photoluminescence.
The ion bombardment of the metal cathode needed to sustain a glow discharge does not generate secondary electrons efficiently. The yield from a typical low work function surface is less than 10%.
Furthermore, where secondary emission is used to generate charge carriers in a small cell, the number of carriers is depleted quickly because of high diffusion losses to the walls of the cell.
Proposals have also been made for planar displays incorporating a matrix of field emitters, such emitters being of the class of thin film structures incorporating microscopic points, edges or discontinuities, which give rise to room temperature free electron emission when a gate or electrode in close proximity is charged to a positive voltage, generally in the range of 10 to 100 V. The emitted electrons are then accelerated towards a phosphor layer, where they cause cathodoluminescence, the same light producing mechanism as in a CRT.
Phosphors, however, have a relatively low efficiency (about 1%) at low cathodoluminescent voltages, of about 400 volts, employed so far. Attempts to increase efficiency by increasing anode voltage to kilovolt levels have met with problems in fabricating displays capable of operating at these voltages.